directory-layout.cabal: add missing test files to tarball

> Linking dist/build/doctest/doctest ...
> Preprocessing test suite 'spec' for directory-layout-0.6.1.0...
>
> test/System/Directory/Layout/InterpreterSpec.hs:18:18:
>     Could not find module ‘SpecHelper’
>     Use -v to see a list of the files searched for.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>